# std::future<T>::valid

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< cpp‎ | thread‎ | future

bool valid() const noexcept; |  | (since C++11)

Checks if the future refers to a shared state.

This is the case only for futures that were not default-constructed or moved from (i.e. returned by std::promise::get_future(), std::packaged_task::get_future() or std::async()) until the first time get() or share() is called.

The behavior is undefined if any member function other than the destructor, the move-assignment operator, or `valid` is called on a `future` that does not refer to shared state (although implementations are encouraged to throw std::future_error indicating `no_state` in this case). It is valid to move from a future object for which `valid()` is false.

### Parameters

(none)

### Return value

true if *this refers to a shared state, otherwise false.

### Example

Run this code

    #include <future>
    #include <iostream>

    int main()
    {
        std::promise<void> p;
        std::future<void> f = p.get_future();

        std::cout << std::boolalpha;

        std::cout << f.valid() << '\n';
        p.set_value();
        std::cout << f.valid() << '\n';
        f.get();
        std::cout << f.valid() << '\n';
    }

Output:

    true
    true
    false

### See also

wait | waits for the result to become available (public member function)

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